1. Field of the Invention
The present invention generally relates to a light modulator for modulating a laser beam and, more particularly, to a high speed light modulator of a kind used in a high speed optical fiber communication system. The present invention also relates to a method of manufacturing such light modulator.
2. Description of the Prior Art
In the high speed optical fiber communication system, a considerable amount of data is transmitted by the use of semiconductor laser beams and optical fibers. In order to cope with this feature, the semiconductor laser beams are required to be modulated at a high speed. With the conventional direct modulation system in which the electrical current injected into a single-mode semiconductor laser is modulated to provide the modulated output laser beam, the change in wavelength resulting from change in density of injected carriers (i.e., wavelength chirping) is so substantial that the conventional direct modulation system cannot be used in high-speed modulation of 10 Gbps or higher.
In view of the foregoing, as an alternative to the direct modulation system, the external modulation system has come to be the cynosure of those concerned, in which a light modulator having a low chirping and disposed externally of a semiconductor laser is utilized to modulate the laser beam while the current injected into the semiconductor laser is fixed. The combined modulator and laser assembly in which a light modulator, a single-mode semiconductor laser and an isolator separating the light modulator and the semiconductor laser from each other are integrated together on a single chip is shown by 60 in FIG. 7. Since no circuit is required between the modulator and the laser, the combined modulator and laser assembly 60 shown therein has a high practical utility and is extremely important as a key device for optical fiber communication of a large amount of data.
The light modulator will now be described. As shown in FIG. 8A, the light modulator 70 includes an InP semiconductor substrate 52 on which a semiconductor mesa layer 56 of a predetermined width containing a light absorption layer 51 and a semiconductor bonding pad layer 55 are formed. The laser beam inputted to the light modulator 70 is modulated by the light absorption layer 51. More specifically, by applying a voltage to the bonding pad electrode 55a, an electric field is applied from an electrode 54, covering the semiconductor mesa layer 56, to the light absorbing layer 51, and by shifting the absorption wavelength of the light absorbing layer 51, the input laser beam is modulated.
As shown in FIG. 8B, a groove 57 is formed between the semiconductor mesa layer 56 and the semiconductor bonding pad layer 55 for separating the semiconductor layers 55 and 56 from other semiconductor layers. The semiconductor mesa layer 56, the semiconductor bonding pad layer 55 and the groove 57 have their respective surfaces covered by a continuous insulating film 53. The bonding pad electrode 55a and the electrode 54 are formed by a metallic film continuously covering the insulating film 53 while the electrode 54 is held in ohmic contact with the semiconductor mesa layer 56 through an opening in the insulating film 53.
The conventional method of manufacturing the conventional light modulator is shown in FIGS. 9A to 9C. Referring first to FIG. 9A, a predetermined crystalline layer is epitaxially grown on the InP substrate 52 to form the semiconductor mesa layer 56 of the predetermined width including the light absorption layer 51, the groove 57 and the semiconductor bonding pad layer 55. Then, as shown in FIG. 9B, the insulating film 53 of SiO.sub.2 having a film thickness of about 4000 .ANG. is formed so as to cover the entire surface of the InP substrate 52. After a window for the ohmic contact has been formed in an upper surface of the semiconductor mesa layer 54 including the light absorption layer 51, the metallic film is formed at a predetermined location as shown in FIG. 9C to complete the bonding pad electrode 55a and the electrode 54.
In order for the light modulator to be used for high-speed modulation, it is necessary to reduce the static capacitance (hereinafter referred to as a "parasitic static capacitance") between surface electrodes (the bonding pad electrode 55a and the electrode 54) and a rear surface electrode. The parasitic static capacitance of the light modulator is expressed by the sum of the parasitic static capacitance of the mesa layer 56 plus the parasitic static capacitance of the bonding pad layer 55. In order to reduce the parasitic static capacitance of the light modulator, attempts have currently been made to minimize the surface area of each of the mesa layer 54 and the bonding pad layer 55 by forming the groove 57 therebetween.
It has, however, been found that considering the chirping of light that is propagated by the light absorption layer 51, the width of the mesa layer 56 can only be reduced to a certain limited dimension. Also, considering the bonding surface area of the bonding wire, the size of the bonding pad layer 55 is limited to about 50.times.50 .mu.m. Thus, the approach to reduce the surface area of the mesa layer 56 and the bonding pad layer 55 in an attempt to reduce the parasitic static capacitance is limited and, therefore, a sufficiently high-speed modulation characteristic has been difficult to accomplish.